MicroChip 2

1
MicroChip 2

• UNCA ECE 406
• April 18, 2007

2
Indirect register access

• FSR register
• Register 04h
• A pointer to another register
• INDF
• Register 00h
• Value of the target register
• Setting FSR to INDF is possible
• But a little silly

3
Array access problem

• Suppose 43h has the address of array A
• And 57h has an index I
• How does you
• AI AI 15
• This is useful only with many registers
• And necessary if you cant address all those file
  registers directly from an instruction

4
Array access solution

• MOVLW 43h W A0
• ADDWF 57h,W W AI
• MOVWF FRS FRS AI
• MOVF INDF,W W AI
• ADDLW 15 W AI15
• MOVF INDF,F AI AI15

5
PIC I/O

• Some pins are available for
• Digital input
• Digital output
• Analog input
• Analog output
• External clocks
• Processor reset
• Programming device
• Wide variations among processes
• Setting the mode isnt easy

6
In PIC10F20X 1

• Some pins must be set with programming
• MCLR (Master Clear) function
• OPTION register is set when programmed
• Can dedicate a pin to external clock
• Otherwise the processor cant run

7
In PIC10F20X 2

• Analog available in PIC10F206
• Register CMDCON0 (07h)
• Turns on/off an input comparator
• Enables comparator output

8
In PIC10F20X 3

• TRISGPIO register
• Tri-State General Purpose I/O
• Hidden in PIC10F
• Accessed with TRIS instruction
• Set to 1 for input
• Clear to 0 for output
• Written with TRIS instruction
• From contents of W
• TRIS GPIO is required for some reason

9
Doing I/O

• Register GPIO
• Read and write
• Thats all
• But it may be interesting if TRIS isnt set right

10
Subroutines

• Be careful about how you use registers
• Probably want to use W
• For single parameter
• And for return
• But unlikely on PIC10F20X
• Small stack for return address
• Only holds two on the PIC10F20X

11
Call / Return instructions

• CALL addr
• Pushes PC1 on stack
• Replaces PC with addr
• Except addr can only be eight bits
• Other bits may come from STATUS register
• Whew.
• RETURN
• Not supported on PIC10F20X
• But does the obvious on other processors

12
A different Return

• RETLW k
• Return with Literal from W
• Sets W with k
• Sets PC to top of stack
• Useful for lookup tables

13
Table lookup

• Store a table in instructors in program memory
• SQUARES RETLW 0
• RETLW 1
• RETLW 4
• RETLW 9
• RETLW 16
• RETLW 25

14
Square lookup

• SQUAREIT ADDLW SQUARES
• W now points to the right place
• MOVWF PCL
• Go there and return
• On most processors youd also want to set some
  bits in the STATUS register

15
I/O pin selection

• Choose your pins
• VDD
• VSS
• GP0 DIN
• GP1 CLK
• GP2 DOUT
• GP3 RST

16
Configuration word

• Configuration word p 41
• Bit 4 0
• GP3 is digital I/O
• Bit 2 0
• Watchdog time disabled
• Not addressable during device operation
• Could set GP3 as MCLR
• For reset

17
Useful definitions

• Choose registers
• STATE 0, 1, 2
• NXTST
• LSTIN DIN on last poll
• Make useful bit vectors
• BDIN 01h
• BCLK 02h
• BDOUT 04h
• BRST 08h